The present application relates to a recording apparatus and a reproducing apparatus that perform recording and reproducing on a recording medium having a reference plane on which a position guide is formed and a planar recording layer on which no position guide is formed, and recording and reproducing methods thereof.
As disclosed in Japanese Patent Application Publication No. 2011-123978, as an optical recording medium on which signals are recorded and from which signals are reproduced by irradiation of light, a so-called optical disc recording medium (also hereinafter simply referred to as an optical disc) such as a Compact Disc (CD), a Digital Versatile Disc (DVD), and a Blu-ray Disc (BD) (registered trademark) have become widespread.
In the related art, a large recording capacity has been achieved by improving information recording density in the optical disc. Specifically, a technique of reducing a formation pitch of a track serving as a pit string or a mark string, that is, improving recording density in a radial direction, and a technique of improving a recording density of a line direction (a direction perpendicular to the radial direction) by pit or mark size reduction have been adopted.
On the other hand, when a recording capacity is increased, a technique of increasing the number of recording layers (recording planes) is also effective, and a multilayer disc such as a disc of two layers or a disc of three or more layers has been proposed and come into practical use even at present.
However, in a recordable multilayer disc widely used at present, a recording layer on which a position guide (for example, a wobbling groove or the like) is formed as in the case of a single-layer disc is laminated.
When a multilayer disc having a plurality of recording layers with the above-described position guide is formed, it is necessary to perform a pattern transfer process using a stamper for lamination of each recording layer. Thus, costs increase due to an increase in the number of processes as compared with the case of a normal single-layer disc or two-layer disc.
In addition, when a failure in the pattern transfer process is considered, the yield is also deteriorated as compared with the single- to two-layer disc, and hence there is also a problem in that costs increase.
This applicant has proposed a multilayer recording medium 100 as illustrated in FIG. 35 as a multilayer recording medium having three or more recording layers.
As illustrated in FIG. 35, on the multilayer recording medium 100, a cover layer 101, a recording-layer formation region 102 (in which the number of recording-layer laminations is five of L1 to L5) in which a plurality of recording layers L are formed, an adhesive layer 103, a reflective film 104, and a substrate 105 are formed in order from an upper layer side.
Here, the “upper layer side” used here refers to an upper layer side when a surface on which laser light from a recording/reproducing apparatus that performs recording and reproducing on the multilayer recording medium 100 is incident is used as an upper side.
Within the recording-layer formation region 102, the recording layers L are formed by semi-transparent recording films. Intermediate layers are inserted between the semi-transparent recording films. It should be noted that no position guide is formed on each recording layer L (semi-transparent recording film) according to formation of a groove, a pit string, or the like as illustrated in the drawing. That is, each recording layer L is formed in a planar shape.
On a lower layer side of the recording-layer formation region 102, the reflective film 104 is formed via the adhesive layer (intermediate layer) 103 formed of a necessary adhesive material.
A position guide for guiding a recording/reproducing position is formed on the reflective film 104. When the position guide is formed on the reflective film, it means that the reflective film is formed on an interface on which the position guide is formed.
Specifically, in this case, the position guide is formed on one surface side of the substrate 105 in the drawing and hence a concave/convex cross-sectional shape as illustrated in the drawing is provided. The reflective film 104 is formed on a surface on which the concave/convex cross-sectional shape of the substrate 105 is provided and hence the position guide is formed on the reflective film 104. The substrate 105 is generated by injection molding or the like using a stamper for providing the concave/convex cross-sectional shape as the above-described position guide.
Here, it is possible to record information (absolute position information: radial position information and rotation angle information) representing an absolute position in a direction parallel to a recording in-plane direction of the multilayer recording medium 100 by forming the above-described position guide as in a current recordable disc. For example, this absolute position information can be recorded by modulation of a meandering (wobble) cycle of the groove when the position guide is formed in the groove, and recorded by modulation of a pit length or formation interval when the position guide is formed in the pit string.
In a state in which no position guide is formed on each recording layer L as described above, the recording position on each recording layer L is controlled based on reflected light from the reflective film 104 on which the position guide is formed as will be described later.
In this sense, the reflective film 104 (reflective surface) on which the position guide is formed is referred to as a “reference plane Ref.”
According to the above-described multilayer recording medium 100, a process of forming the position guide necessary for formation of each recording layer L is unnecessary, and the multilayer recording medium can be implemented at a low cost.
Here, a specific recording technique for the multilayer recording medium 100 according to the above-described structure will be described.
The multilayer recording medium 100 is irradiated with recording-layer laser light to be radiated by targeting the recording layer L.
To implement position control during recording in the recording-layer laser light, laser light (hereinafter referred to as servo laser light) for performing position control based on the position guide in the reference plane Ref is also radiated to the multilayer recording medium 100.
Specifically, the recording-layer laser light and the servo laser light are radiated to the multilayer recording medium 100 via a common objective lens (objective lens 110) as illustrated in FIG. 35.
During recording of a mark targeting a recording layer L, the servo laser light as illustrated in the drawing is radiated to be focused on the reflective surface (the reference plane Ref) of the reflective film 104, and position control of the objective lens 110 is performed according to a tracking error signal obtained based on the reflected light (that is, a tracking servo is applied).
Thereby, it is possible to cooperatively control a position in a tracking direction of recording-layer laser light radiated via the same objective lens 110.
On the other hand, position control during reproduction can be implemented as follows.
Because a mark string (that is, a recorded track) is formed on a recording layer L during reproduction, it is possible to apply a tracking servo by only recording-layer laser light that targets the mark string. That is, the tracking servo can implement the position control of the objective lens 110 according to a tracking error signal obtained based on reflected light of the recording-layer laser light.
Here, when the above-described position control technique is adopted, the reflectance of the recording-layer laser light is forced to be increased on a reference plane Ref from which reflected light of servo laser light should be obtained if light having the same wavelength as the recording-layer laser light is used as the servo laser light. That is, there is a concern in that the number of stray light components is increased and reproduction performance is significantly deteriorated.
Thus, the servo laser light and the recording-layer laser light have different wavelengths, and a reflective film having wavelength selectivity is used as the reflective film 104 forming the reference plane Ref.
As a specific example, the wavelength of the recording-layer laser light is about 405 nm as in the case of the BD, and the wavelength of the servo laser light is about 650 nm as in the case of the DVD. As the reflective film 104, a wavelength-selective reflective film that selectively reflects light of the same wavelength band as the servo laser light and transmits or absorbs light by the other wavelengths is used.
According to the above-described configuration, it is possible to prevent an unnecessary reflected light component of the recording-layer laser light from being generated from the reference plane Ref and secure a good signal to noise ratio (S/N).
Incidentally, in the multilayer recording medium 100 in which a position guide such as a groove is not formed on the recording layer L, a seek for a recording start position during recording is performed using address information recorded on the reference plane Ref.
Specifically, during recording for the recording layer L, a recording start address on the reference plane Ref is specified based on a write command. First, according to the servo laser light, a seek operation is performed for a recording start address on the reference plane Ref is performed. According to the seek completion, recording by the recording-layer laser light is started. Thereby, it is possible to start recording of data from a position corresponding to the above-described recording start address on the recording layer L.
In addition, even for reproducing of information recorded on the recording layer L of the multilayer recording medium 100, first, the seek using an address on the reference plane Ref is performed. Specifically, the seek operation by the servo laser light is performed by targeting a reproduction start address on the reference plane Ref specified based on a read command.
After the seek operation based on the above-described address of the reference plane Ref has been performed, tracking servo control on the objective lens 110 is switched from servo control based on reflected light of the servo laser light to servo control based on reflected light of the recording-layer laser light. Thereby, it is possible to cause a beam spot of the recording-layer laser light to follow a track in the vicinity of the reproduction start position on the recording layer L.
Then, by reading the address information recorded on the recording layer L, the movement to a predetermined reproduction start position is possible and data reproduction from the reproduction start position can be started.
Although the outline of the recording/reproducing operation on the multilayer recording medium 100 in which no position guide is formed on the recording layer L has been described above, in practice, the occurrence of a deviation in an information recording position due to a spot position deviation between the recording-layer laser light and the servo laser light as will be described below should be considered when the recording/reproducing operation is performed on the multilayer recording medium 100.
Here, when the above-described position control technique is adopted, the deviation in the information recording position in a tracking direction occurs due to a lens shift of the objective lens 110 caused due to the eccentricity of the multilayer recording medium 100, the backlash of a slide mechanism of the optical pickup, or the like.
The lens shift according to the backlash of the slide mechanism used here means that a position of the objective lens 110 during tracking servo control is shifted to absorb its displacement according to rapid (instantaneous) displacement of the position of the optical pickup due to the occurrence of mechanical mechanistic backlash in the slide mechanism during slide spot control.
FIGS. 36A, 36B, and 36C are diagrams illustrating a principle in which the deviation in the information recording position is caused by the lens shift of the objective lens 110.
In FIGS. 36A, 36B, and 36C, FIG. 36A illustrates an ideal state in which the eccentricity of the multilayer recording medium 100 or the backlash of the slide mechanism is absent and the lens shift of the objective lens 110 does not occur, FIG. 36B illustrates the case in which the lens shift of the left direction (for example, assumed to be an outer circumferential direction) of the drawing (referred to as the eccentricity of a positive (+) direction) has occurred, and FIG. 36C illustrates the case in which the lens shift of the right direction (for example, assumed to be an inner circumferential direction) of the drawing (referred to as the eccentricity of a negative (−) direction) has occurred.
Although the case in which the reference plane Ref is formed on an upper layer side of the recording layer L has been illustrated in FIGS. 36A, 36B, and 36C for convenience of illustration, the deviation in the information recording position in the same principle occurs even when the reference plane Ref is formed on a lower layer side of the recording layer L as in FIG. 35 described above.
First, a central axis c of the drawing is a central axis set in designing an optical system, and the center of the objective lens 110 is consistent with the central axis c in the ideal state illustrated in FIG. 36A.
On the other hand, when the lens shift of the + direction has occurred as illustrated in FIG. 36B, the center of the objective lens 110 is shifted in the + direction with respect to the central axis c of the optical system.
At this time, because the servo laser light (patterned light rays in the drawing) is incident by parallel light on the objective lens 110, even when the shift from the central axis c of the objective lens 110 as described above occurs, the position change in the tracking direction of the focus position does not occur.
On the other hand, because the recording-layer laser light (outlined light rays in the drawing) is incident by non-parallel light on the objective lens 110 so as to be focused on the recording layer L formed in a depth position different from the reference plane Ref, the focus position (the information recording position) of the recording-layer laser light is changed by an extent corresponding to a lens shift amount in the + direction (a deviation amount +d in the drawing) as illustrated in the drawing with respect to the shift of the objective lens 110 in the + direction as described above.
In addition, when the lens shift in the − direction has occurred as illustrated in FIG. 36C, the information recording position of the recording-layer laser light is changed by an extent corresponding to a lens shift amount in the − direction (a deviation amount − d in the drawing) as illustrated in the drawing.
The recording/reproducing apparatus for the multilayer recording medium 100 described above with reference to FIG. 35 has the following configuration.
The recording-layer laser light and the servo laser light are radiated via the common objective lens 110.
The focus position of the recording-layer laser light is different from the focus position of the servo laser light.
The tracking servo control of the objective lens 110 is performed to cause the focus position of the servo laser light to follow the position guide formed on the reference plane Ref.
In this configuration, there is a problem in that the deviation in the information recording position of the recording-layer laser light occurs in the tracking direction due to the eccentricity of the disc, the backlash of the slide mechanism, or the like.
At this time, there is also a problem in that the information recording position may overlap between the adjacent guide grooves according to the size of the eccentricity or the like or the setting of the track pitch (the guide groove formation interval). Therefore, it is difficult to correctly reproduce a recording signal.
In addition, although the lens shift of the objective lens 110 that occurs as a main cause of the deviation in the information recording position has been described above, the deviation in the information recording position may also similarly occur due to a disc tilt.
As one measure for avoiding the above-described problems of the information recording position deviation, the track pitch can be widened to be greater than or equal to the change in the information recording position.
However, in this measure, there may be a problem in that the recording capacity is decreased due to the widening of the track pitch.
In addition, as another measure, a technique of configuring a system in which the disc is not detachable can also be provided.
Here, as a cause of the eccentricity, there is an error between an inner diameter of a disc and a clamp diameter of a spindle motor. In processing, it is difficult to completely remove the error therebetween so as to be zero, and hence the eccentricity is inevitable. In addition, even when the error therebetween has been removed so as to be zero, the eccentricity also occurs on the surface because the recording signal center of the reference plane of the disc is not necessarily coincident with the spindle shaft center of the recording apparatus. According to the system in which the disc is not detachable, the influence of the eccentricity is the same and hence it is possible to avoid the problem that recording positions overlap. Thereby, the track pitch can be reduced and hence the recording capacity can be increased by the reduction.
However, of course, because it is difficult to replace the disc in this technique, it is difficult to replace only the disc, for example, when the disc is defective. Further, it is difficult for another recording apparatus to read data recorded on a given apparatus. In other words, in this sense, the convenience is lost.
Therefore, as an effective technique for avoiding this problem, a so-called adjacent track servo (ATS) may be adopted. Originally, the ATS has been studied for a self-servo track writer (SSTW) in a hard disk drive.
FIG. 37 is a diagram illustrating the ATS.
As illustrated in the drawing, in the ATS, a recording spot Swr and an ATS spot Sats are configured to be formed on the recording layer L. The spot Swr and the spot Sats are formed by radiating source light rays to the recording medium via the common objective lens. At this time, the distance between the spots is fixed at a predetermined length.
In the ATS, the tracking servo is applied according to the ATS spot Sats by setting the recording spot Swr as a preceding spot (that is, the outer circumference side when a traveling direction of recording is Inner Circumference→Outer Circumference), setting the ATS spot Sats as the following spot, and targeting a mark string formed by the recording spot Swr. As a result, the tracking servo control of the objective lens causes the ATS spot Sats to follow a track one track before formed by the recording spot Swr.
According to the above-described ATS, because the track pitch can be constant at a distance between spots S, there is no problem of the tracks overlapping (the information recording positions overlapping) due to the influence of the eccentricity or the like. That is, it is not necessary to widen the track pitch marginally or configure the system in which the disc is not detachable by considering the deviation in the information recording position caused by the eccentricity or the like as described above.